Patent GB-1493861-A (GB)
Title: Method and apparatus for detecting defects in photomasks
IPC: G01B11/24
Filed: 1975-02-04
Abstract: 1493861 Testing photomasks WESTERN ELECTRIC CO Inc 4 Feb 1975 [6 Feb 1974] 4688/75 Heading G1A A photomask, e.g. for making integrated circuits, consisting of an opaque pattern on a light transmissive substrate, is tested for defects by scanning it with a coherent light spot whose diameter is less than the size of the smallest feature of the pattern; the resulting diffraction pattern is analyzed using photo-detectors so as to distinguish the desired straight edges and corners of the pattern from defects such as spots and rough edges. In Fig. 1 the mask 14 is scanned by a beam from laser 11, scanning being effected by mirror 13. The mask and mirror are both one focal length away from a lens 12; a further lens 15 collects transmitted and diffracted light from the mask and passes it to a de-scanning mirror 16. The latter scans at the same frequency as mirror 13 but is 180 degrees out of phase, so that it passes to lens 17 a diffraction pattern which is stationary, but whose intensity varies with time. Lenses 17 and 19, with aperture 18, remove components due to scattering at the lens surfaces, and the pattern is split into three concentric portions by a device shown in more detail in Fig. 2. A central mirror 31 directs the central portion of the pattern to detector 22 via lens 21; an annular mirror 32 directs an annular part of the pattern to detector 24 via lens 23, and the rest of the pattern passes to twelve detectors 26 via mirrors 25. Comparison of the detector signals shows whether the pattern is the sort associated with a section of a sound mask or with a defect. The process may be speeded by using a second light scanning spot. Alternative arrangements for measuring different parts of the diffraction pattern are disclosed. An array of detectors may be placed just beyond lens 19; or the de-scanning mirror may be replaced by an array of detectors on which the focused beam from lens 15 falls directly. In this latter arrangement the deflection at mirror 13 must be small. The areas of the diffraction pattern which are measured depends on the type of photomask being inspected. For a Manhattan mask, i.e. one having only orthogonal features, the diffraction pattern is analyzed with an array of detectors as in Fig. 7. The features of the mask in one-direction cause regions A and B(0) to be illuminated, while those in the orthogonal direction cause A and B(90) to be illuminated. A corner illuminates A, B(0) and B(90). Regions C are illuminated by defects, and partly by non-sharp corners. To remove the influence of the latter on defect detection, any simultaneous illumination of B(0) and B(90) which occurs and therefore represents a corner, is subtracted from the illumination of C. For photomasks having other features as well as Manhattan ones use is made of concentric parts of the diffraction pattern, e.g. by the detector array of Fig. 8. All sound edges and corners produce the same ratio of illumination of areas B and C, whereas defects produce a different ratio. The incorrect ratio thus indicates a defect. This is independent of light intensity, but because defects can give rise to a high illumination, they can also be detected by thresholding the signals from detector B or C. For photomasks where the diffraction pattern intensity is low the array of Fig. 9, which is equivalent to Figs. 1 and 2, is used. A computer finds the ratio of the instantaneous maximum and minimum illumination of the outer detectors C1-C12; this ratio is high for sound features and low for defects.